A photoelectric detector for detecting a quantity of an incident light volume has a photodiode for generating a charge response to the quantity of the incident light volume, and outputs electric signals in response to the quantity of charge outputted from the photodiode. Such a photoelectric detector is described in, for example, Japanese Unexamined Patent Publication No. Hei-5-21507. FIG. 7 is a configurational view of a prior art photoelectric detector described in the above-described patent publication.
In the prior art photoelectric detector, light is made incident into a photodiode 1, and a current is outputted from the photodiode 1. The current is inputted into an integrating circuit composed of an amplifier 3 and an integrating capacitor 4, and charge is accumulated in the integrating capacitor 4. On the other hand, the accumulated charge is dumped by a certain quantity per time by a series of opening and closing actions of respective switches in a dump circuit consisting of switches S1, S2 and a capacitor Cd. Also, an output signal Vout from the above-described integrating circuit is compared with a prescribed signal Vthreshold by a comparison circuit 7. Where the former is greater than the latter, a series of opening and closing actions of the switches S1 and S2 of the dump circuit are carried out by an instruction from a logic circuit 6, wherein the charge accumulated in the integrating capacitor 4 of the integrating circuit is dumped only by a certain quantity.
And, the number of times of dumping actions per unit time is counted by a counting circuit 8. The counted value is outputted as a digital value that expresses the intensity of light made incident into the photodiode 1. That is, in the photoelectric detector, the A/D converting means is composed of the logic circuit 6 and counting circuit 8, etc.